Rollback
hg rollback
Roll back the last transaction in a repository.
When committing or merging, Mercurial adds the changeset entry last. Mercurial keeps a transaction log of the name of each file touched and its length prior to the transaction. On abort, it truncates each file to its prior length. This simplicity is one benefit of making revlogs append-only. The transaction journal also allows an undo operation.
Help text: http://www.selenic.com/mercurial/hg.1.html#rollback